Touch panels have been widely known as a device for detecting and outputting the position on a touch screen, which is indicated by an indicator such as a user's finger or a pen (hereinafter may be referred to as “touch position”). A plurality of detection methods for detecting the touch position in the touch panel have been known. Examples of capacitive touch panels include a projected capacitive touch panel.
The projected capacitive touch panel can detect the touch position even when a surface of the touch screen on the side of the user (hereinafter may be referred to as “front surface”) is covered with a protective plate such as a glass plate having a thickness of about a few mm. The projected capacitive touch panel has the protective plate disposed on the front surface and has no movable part, and therefore, is advantageously robust and has a long life.
The touch screen of the projected capacitive touch panel is configured of detection column wires for detecting a coordinate of the touch position in the column direction and detection row wires for detecting a coordinate of the touch position in the row direction (for example, refer to Patent Document 1). In following description, the detection column wires and the detection row wires may be collectively referred to as “detection wires”.
Patent Document 1 discloses a touch pad system corresponding to the touch panel. The touch pad system disclosed in Patent Document 1 includes, as detection wires for detecting an electrostatic capacitance (hereinafter may be referred to as merely “capacitance”), a first series of conductive elements formed on a thin dielectric film and a second series of conductive elements formed above the first series of conductive elements via an insulating film. There is no electrical contact between the conductive elements, and when viewed in the normal line direction from the front surface, crossed portions where the first series of conductive elements overlap the second series of conductive elements without electrical contact are formed.
By detecting a capacitance formed between an indicator such as a finger and the conductive elements that are the detection wires (hereinafter may be referred to as “touch capacitance”) by use of a detection circuit, coordinates of the touch position of an indicator are identified. Relative values of detection capacitances of one or more conductive elements can interpolate the touch position between the conductive elements.
In following description, a member in which the detection column wires and the detection row wires are disposed on a transparent dielectric substrate is referred to as “touch screen”, and a device in which a detection circuit is connected to the touch screen is referred to as “touch panel”. A region where the touch position can be detected on the touch screen is referred to as “operational region”.
To thoroughly detect the touch position of the indicator in the operational region of the touch screen, the detection wires need to be densely disposed in the operational region. To densely dispose the detection wires in the operational region, it is required to avoid the problem that the detection wires may be viewable to the user.
When the detection wires are formed of transparent conducting films such as indium tin oxides (abbreviated as ITO), for example, the detection wires are unlikely to be viewable to the user. However, the transparent conducting film such as ITO have a relatively high electric resistance (hereinafter may be referred to as merely “resistance”), which is disadvantageous to increase the size of the touch screen. Further, since the transparent conducting film such as ITO has a relatively low optical transmittance (hereinafter may be referred to as merely “transmittance”), when the touch screen is illuminated from the back surface side, that is, the opposite side to the user, during use in a liquid crystal display (abbreviated as LCD) or the like, a relatively large amount of light is needed, which is disadvantageous to reduce power consumption.
The detection wires may be also made of metal having a low resistance, such as silver and aluminum. Although the resistance of the detection wires can be decreased by using wires made of metal (hereinafter may be referred to as “metal wires”) as the detection wires, the metal wires are opaque and thus, can be easily viewed. To reduce the visibility of the metal wires and increase the transmittance of the touch screen, the metal wires need to be made thin.
When thinned metal wires are used as the detection wires and are densely disposed in the operational region of the touch screen, a parasitic capacitance between the detection column wires and the detection row wires (hereinafter may be referred to as “inter-wire capacitance”) largely increases, causing failures including an increase in wiring delay.
The wiring delay can be somewhat mitigated by decreasing the resistance of the wires. Patent Document 2 discloses a technique of decreasing the resistance of the wires to mitigate the wiring delay.
In a touch screen disclosed in Patent Document 2, detection column wires and detection row wires are linear and thin metal wires arranged in a zigzag manner to reduce both of resistance and inter-wire capacitance.
In the touch screen disclosed in Patent Document 2, the plurality of detection row wires extending substantially in the row direction are electrically connected to form row-direction bundle wires, and the plurality of detection column wires extending substantially in the column direction are electrically connected to form column-direction bundle wires. This enables uniform detection of a touch capacitance including the capacitance between the indicator such as a finger and the detection row wires and the capacitance between the indicator and the detection column wires.
However, in the touch screen disclosed in Patent Document 2, the transmittance locally lowers in the area where the thin metal wires are disposed. Accordingly, when the touch screen is used in combination with a display element disposed opposed to the back surface of the touch screen, uneven luminance and uneven display such as moire occur in a display screen of the display element, and are often viewed as failures to the user. When a picture is placed opposed to the back surface of the touch screen and the touch screen is used as a digitizer or tablet, uneven luminance occurs in the picture, and is often viewed as a failure to the user.
Patent Document 3 discloses a technique of reducing such uneven luminance and uneven display (hereinafter collectively referred to as “uneven display”). In the touch panel disclosed in Patent Document 3, uneven display is reduced by providing independent wires that are not connected to the detection wires in an area surrounded by zigzag detection wires.